This invention is concerned with an apparatus for determining the position of a movable mechanical element and a method of marking a mechanical element, particularly but not exclusively for a mechanical element comprising a piston rod of a ram which is movable relative to a cylinder of the ram.
Means for detecting the position of a piston rod relative to a cylinder of a fluid operable ram are known, but rely on detecting the position of a device actuated by the ram by use of a sensor (such as a potentiometer) placed on the fulcrum of the device, and then inferring the position of the rod from a position of the device activated thereby. However, the rod position measurement obtained may be subject to mechanical deficiencies such as backlash between the rod and the actuated device. Such position detectors may also be mechanical and so are subject to deficiencies and wear themselves. Magnetic marking of a piston rod has been used to give a direct indication of the rod""s position, but such magnetic systems are subject to electromagnetic interference and to demagnetisation of the marking.
It is known to provide optically readable marking on the surface of a mechanically element, and in particular to provide these markings by means of laser marking. However, such laser marking may impair the corrosion resistance of the rod.
It is an aim of the present invention to overcome or reduce at least one or more of the above problems.
According to one aspect of the present invention, we provide an apparatus for determining the position of a movable mechanical element wherein the mechanical element has a marking which is optically readable, the optically readable marking comprising at least part of the mechanical element with a surface having a first reflectivity and a plurality of areas having a second reflectivity, further comprising at least one of the following expedients;
a) said part of the mechanical element being provided with a substrate between the mechanical element and the surface.
b) said areas having a second reflectivity having a width which is generally constant in a direction parallel to the direction of movement of the mechanical element.
Where expedient a) is present, the substrate may be resistant to corrosion.
The substrate may be more resistant to laser marking than said surface having a first reflectivity.
The surface having a first reflectivity may comprise chrome.
The substrate may comprise bronze.
The bronze may comprise 88% to 92% copper and 12% to 8% tin.
The substrate may have a thickness in the range 0.038 mm to 0.051 mm.
The substrate may have a surface finish in the range 0.2 to 0.4 xcexcm.
At least the part of the mechanical element to be provided with a substrate may have a surface finish of less than 0.8 xcexcm.
The first reflectivity may be greater than the second reflectivity.
Where expedient b) is present, the marking may comprise a plurality of code elements, each comprising a plurality of said areas of second reflectivity.
Each code element may encode a numerical value in binary digits.
Each binary digit may be indicated by the distance between two of said areas having a second reflectivity.
Each code element may encode a unique number.
Each code element may comprise a unique part of a pseudorandom binary sequence.
The apparatus may further comprise reading means to provide an output dependent on the position of the mechanical element, said reading means comprising illuminating means, detector means and decoding means.
The illuminating means may comprise a light emitting diode.
The detector means may be disposed to detect light from the illuminating means reflected from said surface having a first reflectivity and said areas having a second reflectivity, the detector means comprising a detector which provides a signal having a value within a first range when light from a part of said surface having said first reflectivity is reflected thereon and having a valve within a second range when light from an area having said second reflectivity.
The detector means may comprise an array having a plurality of said detectors.
The array may provide an intermediate signal to said decoding means, said intermediate signal comprising a signal from each of said detectors.
The decoding means may comprise electronic means to decode said intermediate signal from said detector means to provide said output.
The decoding means may comprise means to detect a detector signal of the intermediate signal corresponding to a transition corresponding to an edge of an area of second reflectivity and means to identify a plurality of binary digits from the separation of a plurality of said transitions.
The decoding means may comprise means to identify a code element from said binary digits and provide said output indicating the position of the mechanical rod dependent on said code element.
The decoding means may further comprise means to identify the detector of the detector array on which a selected one of said transitions falls, to provide a fine position.
The apparatus may further comprise focusing means to focus light reflected from said surface said of mechanical element or said areas onto said detector means.
The mechanical element may comprise a piston rod of a fluid operated ram.
According to a second aspect of the invention, we provide a method of marking a mechanical element comprising the steps of providing a surface having a first reflectivity on said mechanical element, and providing a marking thereon with a laser, said marking comprising a plurality of areas having a second reflectivity, further comprising one of the following expedients;
a) prior to providing the mechanical element with said surface having first reflectivity, the step of providing the mechanical element with a substrate on which said surface is then provided,
b) marking said areas having a second reflectivity such that said areas have a width which is generally constant in a direction parallel to the direction of movement of the mechanical element.
According to a third aspect of the invention we provide a mechanical element marked by a method according to the second aspect of the invention.
The apparatus according to the first aspect of the invention may comprise a mechanical element according to the third aspect of the invention.